Photoacoustic spectroscopy (PAS) is an analytical method that involves stimulating a sample with modulated light and detecting the resulting sound waves emanating from the sample. A photoacoustic measurement can be made as follows. First, light is used to stimulate molecules within a sample. Such stimulation can include, for example, absorption of the light by the molecule to change an energy state of the molecule. As a result, the stimulated molecule enters an excited state. Optical excitation is followed by the energy transfer processes (relaxation) from the initially excited molecular energy level to other degrees of freedom, in particular translational motion of the fluid molecules. During such relaxation, heat, light, volume changes and other forms of energy can dissipate into the environment surrounding the molecule. Such forms of energy cause expansion or contraction of materials within the environment. As the materials expand or contract, sound waves are generated.
In order to produce identifiable sound waves, or photoacoustic signals, the light is pulsed or modulated at a specific resonant acoustic or modulation frequency f (having a modulation period 1/f), sometimes also referred to herein as ω. The sample environment can be enclosed and may be constructed to resonate at the modulation frequency. An acoustic detector mounted in acoustic communication with the sample environment can detect changes occurring as a result of the modulated light stimulation of the sample. Because the amount of absorbed energy is proportional to the concentration of the absorbing molecules, the acoustic signal can be used for concentration measurements.
In typical PAS, a resonant acoustic cavity or sample cell with a quality factor Q is used to isolate and amplify sound wave signals, thereby increasing sensitivity of detection. The light intensity or wavelength is modulated at f. The absorbed energy is accumulated in the acoustic mode of the sample cell during Q oscillation periods. Hence, the acoustic signal is proportional to the effective integration or energy accumulation time t, where t=Q/f. Most often the Q factor is in the range 40-200 and f=1,000-4,000 Hz. Thus, for example, Q may equal 70 and f=1250 Hz, with the result that t=0.056 s.
Typically, only a narrow range of wavelengths of light is introduced into a sample. Such narrow range of wavelengths can be formed by, for example, a laser. Utilization of only a narrow range of wavelengths can enable pre-selected molecular transitions to be selectively stimulated and studied. In some instances the species of interest may have an absorption spectrum that is sufficiently distinct that a meaningful measurement can be made simple by carrying out PAS at that wavelength.
In other instances, however, such as when the sample contains more than one molecular species with overlapping absorption spectra, it is difficult or even impossible to measure or detect the presence of one of the species using PAS. When the two (or more) spectra are overlaid, there is no way to identify the spectrum attributable to each species in the resulting spectrum. Hence, it is desirable to provide a PAS technique that will allow distinction between gaseous species in a multi-component mixture even when their absorption spectra overlap.